A joint of a rail is a portion that is most easily damaged in the rail and incurs maintenance cost. Further, a joint of a rail is a main generation source of noise and vibration that are generated during the passage of a train. The speed of passenger rail services and the load of freight trains are increasing at home and abroad. From this situation, a technique for manufacturing a long rail having a length of 200 m or more by continuously connecting joints of rails by welding is becoming widespread.
As main methods of welding a rail joint, there are flash butt welding (for example, see Patent Document 1), gas pressure welding (for example, see Patent Document 2), enclosed arc welding (for example, see Patent Document 3), and Thermit welding (for example, see Patent Document 4).
When a joint of a rail is welded, stress is concentrated in the vicinity of the neutral axis of a rail weld zone. Accordingly, it is necessary to frequently replace a rail in order to prevent fatigue cracks from being generated. FIG. 7A shows an aspect where a fatigue crack 51 extending in the horizontal direction is generated in the vicinity of the neutral axis of a rail weld zone 50 and a brittle crack 52 is growing toward a rail head portion and a rail foot portion. An aspect where the fatigue crack 51 is generated from the vicinity of the neutral axis of the rail weld zone 50 as an origination and the brittle crack 52 then penetrates the rail web portion in the thickness direction is found from FIG. 7B that shows a fracture surface of the crack.
Meanwhile, in this specification, a rail upper portion coming into contact with a wheel is referred to as a “head portion”, a rail lower portion coming into contact with a sleeper is referred to as a “foot portion”, and a portion formed between the head portion and the foot portion is referred to as a “rail web portion”. Further, the upper surface of the head portion is referred to as a “head-top portion”, side surfaces of the head portion are referred to as “head-side portions”, and the back surface of the foot portion is referred to as a “sole portion”.
It is considered that the generation of the fatigue crack is affected by not only an external load condition but also residual stress in the rail weld zone. FIGS. 8A, 8B, and 9 show an example of residual stress distribution generated when a joint of a rail is subjected to flash butt welding. In the graphs of FIGS. 8A, 8B, and 9, a positive direction of the vertical axis represents tensile residual stress and a negative direction of the vertical axis represents compressive residual stress. FIG. 8A shows the residual stress distribution which is generated at a peripheral portion of the rail weld zone in a circumferential direction. From FIG. 8A, it is found that tensile residual stress of the rail web portion is large. Further, FIG. 8B is a view showing tensile residual stress of a middle portion of the rail web portion in the circumferential direction (vertical direction), while a distance from a welding center plane in the axial direction of the rail is represented on the horizontal axis. From FIG. 8B, it is found that tensile residual stress in the circumferential direction (vertical direction) is distributed in the range between the welding center plane and a position that is distant from the welding center plane by a distance of about 25 mm. If a rail weld zone is positioned on the sleeper, compressive stress in the vertical direction acts on the rail web portion during the passage of a train. However, large tensile stress in the vertical direction remains at the rail web portion. Accordingly, while tensile stress is always substantially applied to the rail web portion, the rail web portion repeatedly receives stress. For this reason, fatigue cracks are apt to be generated at the rail web portion. Meanwhile, FIG. 9 shows the residual stress distribution of the peripheral portion of the rail weld zone in the axial direction of the rail. From FIG. 9, it is found that large compressive stress remains at a rail sole portion. If a rail weld zone is positioned between sleepers, tensile stress in the axial direction of the rail acts on the rail sole portion during the passage of a train. However, the tensile stress in the axial direction of the rail and the compressive residual stress in the axial direction of the rail offset each other. Accordingly, while compressive stress is always substantially applied to the rail foot portion, the rail foot portion repeatedly receives stress. For this reason, fatigue cracks are not easily generated at the rail foot portion.
In order to prevent the damage to the rail web portion, a method of rapidly cooling the head portion and the rail web portion of the rail weld zone or the entire rail weld zone, which is in a high-temperature state by welding heat or heat transferred from the outside, is proposed in Patent Document 5 and Patent Document 6. According to this method, it is possible to reduce tensile residual stress that is generated at the rail web portion of the rail weld zone in the vertical direction or to convert the tensile residual stress into compressive stress. Accordingly, it is possible to improve the fatigue resistance of the rail weld zone.
Further, as techniques that improve the fatigue strength of the rail weld zone, there is a method that uses a shot-peening treatment (for example, see Patent Document 7) or the like. In the shot-peening treatment, steel balls which have a diameter of several mm are projected to a material to plastically deform the surface layer of the material, so that the surface layer is subjected to work hardening. That is, it is possible to improve fatigue strength by converting residual stress into compressive stress.
Furthermore, the invention of a device for cooling a rail weld zone is disclosed in Patent Document 8. The device includes an air chamber that cools a head-top surface of the rail weld zone, an air chamber that cools head-side surfaces of the rail weld zone, and air chambers that cool an abdomen portion (rail web portion) and a bottom portion (foot portion) of the rail weld zone. Each of the air chambers is provided with a plurality of nozzles that ejects compressed air, and a nozzle for detecting temperature is provided in the middle of a nozzle group of the air chamber that cools the head-top portion.